RF and IF filters are typically used in RF Communications Transceivers (Transmitters and Receivers) for pre- or post-selection of desired signals. Filters are typically sub-octave and enhance receiver (RX) selectivity by rejecting unwanted signals at image frequencies and other points of spurious sensitivity. For a transmitter (TX), filters reject unwanted spurious and harmonics prior to final RF power amplification. Filters are also used at intermediate frequencies in both up/down conversion architecture to reject unwanted signals, as described above.
In a harsh environment, filter response is required to roll-off rapidly outside the band of interest, necessitating a high order filter. In addition, in-band loss must be minimized through the filter to minimize signal degradation. These two requirements drive precision (less than 2% tolerance), high-Q (Quality factor) inductors and capacitor lumped element implementations. Such elements typically have Q's greater than 100 and are typically required for narrow band or high rejection stopband/selectivity performance. Typically, high performance lumped filters require significant touch labor for assembly and tuning. Typical integrated microwave assemblies (IMA) use LC (inductor-capacitor) filters utilizing air core solenoid wire inductors (air or wrapped coil-forms) and parallel plate capacitors that are hand trimmed and tuned, as surface mount components usually do not have the required precision, nor the necessary Q. The latter is especially pertinent to smaller solenoidal SMT inductors, usually attaining precision between 2% to 5% and Q's less than 100 for optimal small sizes. Other implementations of solenoidal inductors are referenced in U.S. Pat. No. 7,088,215, entitled, “Embedded Duo Planar Printed Inductor”, the entire content of which is hereby incorporated herein. Although providing desirable operations, this approach provides for part of a printed substrate using expensive materials, such as low temperature co-fired ceramic (LTCC). In certain cost sensitive applications, a lower cost material set may be desired with high yield and without added human tuning requirements.
The complexity of hand tuned filters increases exponentially with the order of the filter. In addition, hand tuned lumped filters are packaged to ensure electrical performance and isolation. Highest Q inductors are typically constructed with air cores and require large volumes. Consequently inductor diameters for lower frequency filter applications (below 1 GHz) are large and result in package heights to greater than 0.25 inches. This height is unacceptable for many compact filter requirements. In addition, tight packing densities induce interaction with other inductor elements, reducing performance and increasing tuning complexities. Additional walls are thus typically added for effective isolation and/or rejection, increasing complexity and adding cost and size.